U.S. patent application number 15/883611 was filed with the patent office on 2018-08-02 for nonaqueous electrolyte solution and method for producing nonaqueous electrolyte secondary battery.
This patent application is currently assigned to Toyota Jidosha Kabushiki Kaisha. The applicant listed for this patent is Hiroto Asano, Toshiyuki Kawai, Shinichi Kinoshita, Shigeaki Yamazaki. Invention is credited to Hiroto Asano, Toshiyuki Kawai, Shinichi Kinoshita, Shigeaki Yamazaki.
Application Number | 20180219259 15/883611 |
Document ID | / |
Family ID | 62843539 |
Filed Date | 2018-08-02 |
United States Patent
Application |
20180219259 |
Kind Code |
A1 |
Kawai; Toshiyuki ; et
al. |
August 2, 2018 |
NONAQUEOUS ELECTROLYTE SOLUTION AND METHOD FOR PRODUCING NONAQUEOUS
ELECTROLYTE SECONDARY BATTERY
Abstract
The present disclosure provides a nonaqueous electrolyte
solution that is used in a nonaqueous electrolyte secondary
battery. The nonaqueous electrolyte solution contains a fluorinated
solvent, a predetermined additive A and a predetermined additive B.
A ratio (C.sub.A/C.sub.B) of concentration C.sub.A (mol/L) of the
additive A and concentration C.sub.B (mol/L) of the additive B lies
in a range of 1 to 30.
Inventors: |
Kawai; Toshiyuki;
(Nagoya-shi Aichi-ken, JP) ; Asano; Hiroto;
(Nisshin-shi Aichi-ken, JP) ; Yamazaki; Shigeaki;
(Osaka-shi Osaka-fu, JP) ; Kinoshita; Shinichi;
(Osaka-shi Osaka-fu, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kawai; Toshiyuki
Asano; Hiroto
Yamazaki; Shigeaki
Kinoshita; Shinichi |
Nagoya-shi Aichi-ken
Nisshin-shi Aichi-ken
Osaka-shi Osaka-fu
Osaka-shi Osaka-fu |
|
JP
JP
JP
JP |
|
|
Assignee: |
Toyota Jidosha Kabushiki
Kaisha
Toyota-shi Aichi-ken
JP
Daikin Industries, Ltd.
Osaka-Shi Osaka
JP
|
Family ID: |
62843539 |
Appl. No.: |
15/883611 |
Filed: |
January 30, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 10/0525 20130101;
H01M 10/0568 20130101; H01M 10/0567 20130101; C07D 317/36 20130101;
H01M 2300/0034 20130101; H01M 2300/0025 20130101; Y02E 60/10
20130101; H01M 10/0569 20130101; C07F 5/025 20130101 |
International
Class: |
H01M 10/0568 20060101
H01M010/0568; H01M 10/0567 20060101 H01M010/0567; H01M 10/0569
20060101 H01M010/0569; H01M 10/0525 20060101 H01M010/0525; C07F
5/02 20060101 C07F005/02; C07D 317/36 20060101 C07D317/36 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 1, 2017 |
JP |
2017-017115 |
Claims
1. A nonaqueous electrolyte solution used in a nonaqueous
electrolyte secondary battery, comprising: a fluorinated solvent;
an additive A represented by General Foimula (I): ##STR00005##
wherein, R.sup.1, R.sup.2, and R.sup.3 are selected, each
independently, from the group consisting of hydrogen atoms,
fluorine atoms, methyl groups, fluoromethyl groups and
difluoromethyl groups, and X.sup.1 and X.sup.2 are selected, each
independently, from the group consisting of hydrogen atoms and
fluorine atoms, where at least one from among R.sup.1, R.sup.2,
R.sup.3, X.sup.1, and X.sup.2 is a fluorine atom or a group
comprising a fluorine atom; and an additive B represented by
General Formula (II): ##STR00006## wherein, R.sup.4 and R.sup.5 are
selected, each independently, from among halogen atoms and
perfluoroalkyl groups, and A.sup.+is a cation, wherein a ratio
(C.sub.A/C.sub.B) of concentration C.sub.A (mol/L) of the additive
A and concentration C.sub.B (mol/L) of the additive B in the
nonaqueous electrolyte solution lies in a range of 1 to 30.
2. The nonaqueous electrolyte solution according to claim 1,
comprising, as the additive A, at least one selected from the group
consisting of fluoropropylene carbonate and difluoropropylene
carbonate.
3. The nonaqueous electrolyte solution according to claim 1,
comprising lithium difluorooxalatoborate as the additive B.
4. A method for producing a nonaqueous electrolyte secondary
battery, the method comprising: constructing a battery assembly by
accommodating a positive electrode and a negative electrode inside
a battery case, together with a nonaqueous electrolyte solution;
and subjecting the battery assembly to an initial charging
treatment, wherein the nonaqueous electrolyte solution accommodated
in the battery assembly includes: a fluorinated solvent; an
additive A represented by General Formula (I): ##STR00007##
wherein, R.sup.1, R.sup.2, and R.sup.3 are selected, each
independently, from the group consisting of hydrogen atoms,
fluorine atoms, methyl groups, fluoromethyl groups and
difluoromethyl groups, and X.sup.1and X.sup.2 are selected, each
independently, from the group consisting of hydrogen atoms and
fluorine atoms, where at least one from among R.sup.I, R.sup.2,
R.sup.3, X.sup.I, and X.sup.2 is a fluorine atom or a group
comprising a fluorine atom; and an additive B represented by
General Formula (II): ##STR00008## wherien, R.sup.4 and R.sup.5 are
selected, each independently, from among halogen atoms and
perfluoroalkyl groups, and A.sup.+is a cation, and wherein a ratio
(C.sub.A/C.sub.B) of concentration C.sub.A (mol/L) of the additive
A and concentration C.sub.B (mol/L) of the additive B in the
nonaqueous electrolyte solution lies in a range of 1 to 30.
5. The method for producing a nonaqueous electrolyte secondary
battery according to claim 4, wherein the nonaqueous electrolyte
solution comprises, as the additive A, at least one selected from
the group consisting of fluoropropylene carbonate and
difluoropropylene carbonate.
6. The method for producing a nonaqueous electrolyte secondary
battery according to claim 4, wherein the nonaqueous electrolyte
solution comprises lithium difluorooxalatoborate, as the additive
B.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to Japanese Patent
Application No. 2017-17115 filed on Feb. 1, 2017, the entire
contents of which are incorporated in the present specification by
reference.
BACKGROUND
1. Technical Field
[0002] The present disclosure relates to a nonaqueous electrolyte
solution and to a method for producing a nonaqueous electrolyte
secondary battery.
2. Description of the Related Art
[0003] Nonaqueous electrolyte secondary batteries such as lithium
ion secondary batteries are smaller, lighter and afford higher
energy densities than other existing batteries. Nonaqueous
electrolyte secondary batteries have accordingly come to be used,
in recent years, as power sources for vehicle driving in hybrid
cars, electric cars and the like. Part of the nonaqueous
electrolyte solution in such nonaqueous electrolyte secondary
batteries decomposes generally during initial charging, and a
protective coating film (solid electrolyte interface film: SEI
film) containing decomposition products of the foregoing is formed
on the surface of the negative electrode. The interface between the
negative electrode and the nonaqueous electrolyte solution is
stabilized by such a SEI film, and thus the durability (for
instance cycle characteristics) of the battery can be enhanced.
Examples of related prior art documents include for instance
Japanese Patent Application Publication No. 2007-165125. For
instance, Japanese Patent Application Publication No. 2007-165125
discloses a feature wherein the durability of a battery can be
enhanced by incorporating lithium difluorooxalatoborate (LiDFOB) in
a nonaqueous electrolyte solution, to thereby form a SEI film
containing components derived from LiDFOB, on the surface of the
negative electrode.
SUMMARY
[0004] Electrolyte solutions obtained by dissolving a supporting
salt (for instance a lithium salt) in a carbonate-based solvent
such as ethylene carbonate, propylene carbonate, diethyl carbonate
or the like are used as electrolyte solutions that are utilized in
nonaqueous electrolyte secondary batteries. In terms of further
enhancing the performance of the secondary battery (for instance
increasing energy density), however, electrolyte solutions in which
there are used solvents less prone to oxidizing than such
carbonate-based solvents can be used. Fluorinated solvents
(solvents having fluorine atoms introduced in the molecule) have
been studied as solvents that oxidize less readily than
carbonate-based solvents. However, it has been occasionally
observed that when an additive such as LiDFOB described above is
also used in cases where a fluorinated solvent is utilized as an
electrolyte solution solvent, a good coating film cannot be formed
on the negative electrode, and cycle durability (for instance
capacity retention rate after cycling) is insufficient.
[0005] It is an object of the present disclosure, arrived at in the
light of the above considerations, to provide a nonaqueous
electrolyte solution excellent in oxidation resistance and that
allows realizing high durability. A further object of the
disclosure is to provide a method for producing a nonaqueous
electrolyte secondary battery that is equipped with such a
nonaqueous electrolyte solution.
[0006] In the present specification there is provided a nonaqueous
electrolyte solution used in a nonaqueous electrolyte secondary
battery. The nonaqueous electrolyte solution contains a fluorinated
solvent, an additive A represented by General Formula (I) and an
additive B represented by General Formula (II). A ratio
(C.sub.A/C.sub.B) of the concentration C.sub.A (mol/L) of the
additive A and the concentration C.sub.B (mol/L) of the additive B
in the nonaqueous electrolyte solution lies in the range of 1 to
30.
##STR00001##
Wherein, R.sup.1 to R.sup.3 (i.e., R.sup.1, R.sup.2, and R.sup.3)
are selected, each independently, from the group consisting of
hydrogen atoms, fluorine atoms, methyl groups, fluoromethyl groups
and difluoromethyl groups, and X.sup.1 and X.sup.2 are selected,
each independently, from the group consisting of hydrogen atoms and
fluorine atoms, where at least one from among R.sup.1 to R.sup.3,
and X.sup.1 and X.sup.2, is a fluorine atom or a group containing a
fluorine atom.
##STR00002##
Wherein, R.sup.4 and R.sup.5 in the formula are selected, each
independently, from among halogen atoms and perfluoroalkyl groups,
and A.sup.+ is a cation.
[0007] The cycle durability (for instance capacity retention rate
after cycling) of the nonaqueous electrolyte secondary battery can
be increased by combining the additive A represented by General
Formula (I) and the additive B represented by General Formula (II)
at a specific concentration ratio.
[0008] In some aspects of the nonaqueous electrolyte solution
disclosed herein the solution contains, as the additive A, at least
one carbonate selected from the group consisting of fluoropropylene
carbonate and difluoropropylene carbonate. The effect of enhancing
cycle durability is brought out more readily by an additive A
having such a structure.
[0009] In some aspects of the nonaqueous electrolyte solution
disclosed herein the solution contains lithium
difluorooxalatoborate, as the additive B. The effect of enhancing
cycle durability is brought out more readily by an additive B
having such a structure.
[0010] The present disclosure provides also a method for producing
a nonaqueous electrolyte secondary battery. The production method
includes a step of constructing a battery assembly by accommodating
a positive electrode and a negative electrode inside a battery
case, together with a nonaqueous electrolyte solution, and an
initial charging step of subjecting the battery assembly to an
initial charging treatment. The nonaqueous electrolyte solution
accommodated in the battery assembly contains a fluorinated
solvent, the additive A represented by General Formula (I) and the
additive B represented by General Formula (II), such that a ratio
(C.sub.A/C.sub.B) of the concentration C.sub.A (mol/L) of the
additive A and the concentration C.sub.B (mol/L) of the additive B
in the nonaqueous electrolyte solution lies in the range of 1 to
30. The above production method allows producing a high-performance
secondary battery excellent in cycle durability.
[0011] In some aspects, the nonaqueous electrolyte solution
contains, as the additive A, at least one carbonate selected from
the group consisting of fluoropropylene carbonate and
difluoropropylene carbonate. In other aspects, the nonaqueous
electrolyte solution contains lithium difluorooxalatoborate, as the
additive B.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a diagram illustrating schematically a lithium ion
secondary battery according to an embodiment.
DETAILED DESCRIPTION
[0013] Embodiments of the present disclosure will be explained
below with reference to accompanying drawings. The dimensional
relationships in the figures (length, width, thickness and so
forth) do not reflect actual dimensional relationships. Any
features other than the matter specifically set forth in the
present specification and that may be necessary for carrying out
the present disclosure (for instance, the configuration and
production method of an electrode body provided with a positive
electrode and a negative electrode, the configuration and
production method of a separator, the shape and so forth of a
battery (case), as well as ordinary techniques pertaining to
battery construction) can be regarded as instances of design matter
for a person skilled in the art based on known techniques in the
relevant technical field. The present subject matter can be
realized on the basis of the disclosure of the present
specification and common technical knowledge in the relevant
technical field.
[0014] In the present specification the term "nonaqueous
electrolyte secondary battery" refers to a secondary battery that
is provided with a nonaqueous electrolytic solution (i.e., an
electrolyte solution containing a supporting salt (supporting
electrolyte) in a nonaqueous solvent). The term "lithium ion
secondary battery" refers to a secondary battery that uses lithium
ions as electrolyte ions and that is charged and discharged as a
result of movement of lithium ions between the positive and
negative electrodes. The term electrode active material denotes a
material capable of reversibly storing and releasing a chemical
species (lithium ions in a lithium ion secondary battery) that
constitutes a charge carrier. A nonaqueous electrolyte solution
that is used in lithium ion secondary batteries will be explained
below, but the scope of use of the present disclosure is not meant
to be limited thereto.
[0015] Nonaqueous Electrolyte Solution
[0016] A nonaqueous electrolyte solution according to some
embodiments of the technology disclosed herein is a nonaqueous
electrolyte solution used in lithium ion secondary batteries, the
solution being liquid at normal temperature (for instance at
25.degree. C.). In some embodiments, the solution may be a liquid
at all times within a service temperature range (for instance
-20.degree. C. to 60.degree. C.). Such a nonaqueous electrolyte
solution contains a fluorinated solvent, a supporting salt, an
additive A and an additive B.
[0017] Supporting Salt
[0018] Various materials known to be usable as supporting salts
(lithium salts) in nonaqueous electrolyte solutions of lithium ion
secondary batteries can be used herein, without particular
limitations, as the supporting salt. Examples of supporting salts
include for instance LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6,
LiCF.sub.3SO.sub.3, LiC.sub.4F.sub.9SO.sub.3,
Li(CF.sub.3SO.sub.2).sub.2N, LiC(CF.sub.3SO.sub.2).sub.3,
LiSiF.sub.6, LiClO.sub.4 and the like. The foregoing can be used
singly or in combinations of two or more types. The concentration
of the supporting salt may be set to lie in the range of 0.5 mol/L
to 3.0 mol/L, or from 0.5 mol/L to 2.0 mol/L.
[0019] Fluorinated Solvent
[0020] Various materials usable as nonaqueous solvents in
nonaqueous electrolyte solutions of lithium ion secondary
batteries, and being partially substituted with fluorine (F), can
be used herein as the fluorinated solvent, without particular
limitations. For instance fluorinated cyclic carbonates and
fluorinated linear carbonate may be used as the fluorinated
solvent. Examples of fluorinated cyclic carbonates include for
instance monofluoroethylene carbonate (MFEC), difluoroethylene
carbonate, 4,4-difluoroethylene carbonate, trifluoroethylene
carbonate, trifluoropropylene carbonate (TFPC), perfluoroethylene
carbonate and the like. Examples of fluorinated linear carbonates
include for instance fluoromethylmethyl carbonate,
difluoromethylmethyl carbonate, trifluoromethylmethyl carbonate,
fluoromethyldifluoromethyl carbonate, bis(fluoromethyl) carbonate,
bis(difluoromethyl) carbonate, bis(trifluoromethyl) carbonate,
(2-fluoroethyl)methyl carbonate, ethylfluoromethyl carbonate,
(2,2-difluoroethyl)methyl carbonate, (2-fluoroethyl)fluoromethyl
carbonate, ethyldifluoromethyl carbonate,
(2,2,2-trifluoroethyl)methyl carbonate,
(2,2-difluoroethyl)fluoromethyl carbonate,
(2-fluoroethyl)difluoromethyl carbonate, ethyltrifluoromethyl
carbonate, ethyl-(2-fluoroethyl) carbonate,
ethyl-(2,2-difluoroethyl) carbonate, bis(2-fluoroethyl) carbonate,
ethyl-(2,2,2-trifluoroethyl) carbonate,
2,2-difluoroethyl-2'-fluoroethyl carbonate, bis(2,2-difluoroethyl)
carbonate, 2,2,2-trifluoroethyl-2'-fluoroethyl carbonate,
2,2,2-trifluoroethyl-2',2'-difluoroethyl carbonate,
bis(2,2,2-trifluoroethyl) carbonate, pentafluoroethylmethyl
carbonate, pentafluoroethylfluoromethyl carbonate,
pentafluoroethylethyl carbonate, bis(pentafluoroethyl) carbonate
and the like.
[0021] A combined system of the above fluorinated cyclic carbonate
and the above fluorinated linear carbonate may be used as the
fluorinated solvent. For instance, the mixing ratio of the
fluorinated cyclic carbonate and the fluorinated linear carbonate
may lie in the range of 20:80 to 40:60 by volume.
[0022] The nonaqueous electrolyte solution disclosed herein may
contain a nonaqueous solvent (hereafter also referred to as
non-fluorinated solvent) other than the fluorinated solvent, so
long as the effect of the present subject matter is not impaired
thereby. Examples of such non-fluorinated solvents include for
instance ethylene carbonate (EC), propylene carbonate (PC), diethyl
carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate
(EMC), 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran,
2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, diethylene glycol
dimethyl ether, ethylene glycol dimethyl ether, acetonitrile,
propionitrile, nitromethane, N,N-dimethylformamide, dimethyl
sulfoxide, sulfolane, .gamma.-butyrolactone and the like.
[0023] The amount of the non-fluorinated solvent is 30 vol % or
less, 20 vol % or less, or 10 vol % or less, with respect to the
total volume of the nonaqueous solvent contained in the nonaqueous
electrolyte solution. The technology disclosed herein can be
implemented in a form where the proportion of the fluorinated
solvent in the total volume of the nonaqueous solvent contained in
the nonaqueous electrolyte solution is higher than 90 vol %. The
proportion of the fluorinated solvent may be 95 vol % or higher, 98
vol % or higher, or even 99 vol % or higher, for instance in terms
of increasing oxidation resistance. In some embodiments, a
nonaqueous electrolyte solution may include 100 vol % of the
nonaqueous solvent contained in the nonaqueous electrolyte solution
that is made up of a fluorinated solvent.
[0024] Additive A
[0025] The nonaqueous electrolyte solution disclosed herein
contains, as the additive A, a propylene carbonate derivative
represented by General Formula (I) below.
##STR00003##
Wherein R.sup.1 to R.sup.3 in General Formula (I) (i.e., R.sup.1,
R.sup.2, and R.sup.3) are selected, each independently, from the
group consisting of hydrogen atoms, fluorine atoms, methyl groups,
fluoromethyl groups and difluoromethyl groups, and X.sup.1 and
X.sup.2 are selected, each independently, from the group consisting
of hydrogen atoms and fluorine atoms, the at least one from among
R.sup.1 to R.sup.3, and X.sup.1 and X.sup.2, is a fluorine atom or
a group containing a fluorine atom. In the present specification
propylene carbonate derivative represented by General Formula (I)
encompasses conceptually each geometric isomer of the derivative.
The substitution position of R.sup.1 and R.sup.2 in the propylene
carbonate derivative is herein position 1, the substitution
position of R.sup.3 is position 2 and the substitution position of
X.sup.1 and X.sup.2 is position 3.
[0026] In an example of the additive A, all of R.sup.1, R.sup.2,
R.sup.3, X.sup.1 and X.sup.2 are a hydrogen atom or a fluorine
atom. Examples of such additive A include for instance
3-fluoropropylene carbonate, 3,3-difluoropropylene carbonate,
1-fluoropropylene carbonate, 1,3-difluoropropylene carbonate,
1,3,3-trifluoropropylene carbonate, 1,1-difluoropropylene
carbonate, 1,2-difluoropropylene carbonate,
1,1,3-trifluoropropylene carbonate, 1,2,3-trifluoropropylene
carbonate, 1,1,2-trifluoropropylene carbonate,
1,1,2,3-tetrafluoropropylene carbonate,
1,1,3,3-tetrafluoropropylene carbonate,
1,2,3,3-tetrafluoropropylene carbonate,
1,1,2,3,3-pentafluoropropylene carbonate, 2-fluoropropylene
carbonate, 2,3-difluoropropylene carbonate,
2,3,3-trifluoropropylene carbonate and the like. In some
embodiments, Additive A may include 3-fluoropropylene carbonate and
3,3-difluoropropylene carbonate, in terms of enhancing cycle
durability.
[0027] In another example of the additive A, all of R.sup.1,
R.sup.2, X.sup.1 and X.sup.2 are a hydrogen atom or a fluorine
atom, and R.sup.3 is a methyl group, a fluoromethyl group or a
difluoromethyl group. Examples of such additive A include for
instance 3-fluoro-2-methyl propylene carbonate,
3,3-difluoro-2-methyl propylene carbonate, 1-fluoro-2-methyl
propylene carbonate, 1,3-difluoro-2-methyl propylene carbonate,
1,3,3-trifluoro-2-methyl propylene carbonate, 1,1-difluoro-2-methyl
propylene carbonate, 1,1,3-trifluoro-2-methyl propylene carbonate,
1,1,3,3-tetrafluoro-2-methyl propylene carbonate,
3-fluoro-2-(fluoromethyl) propylene carbonate,
3,3-difluoro-2-(fluoromethyl) propylene carbonate,
1-fluoro-2-(fluoromethyl) propylene carbonate,
1,3-difluoro-2-(fluoromethyl) propylene carbonate,
3-fluoro-2-(difluoromethyl) propylene carbonate,
3,3-difluoro-2-(difluoromethyl) propylene carbonate,
1-fluoro-2-(difluoromethyl) propylene carbonate,
1,3-difluoro-2-(difluoromethyl) propylene carbonate and the
like.
[0028] Other examples of the additive A include instances where
R.sup.1 is a methyl group, a fluoromethyl group or a difluoromethyl
group, and all of R.sup.2, R.sup.3, X.sup.1 and X.sup.2 are a
hydrogen atom or a fluorine atom. Examples of such additive A
include for instance 3-fluoro-1-methyl propylene carbonate,
3,3-difluoro-1-methyl propylene carbonate, 1-fluoro-1-methyl
propylene carbonate, 1,3-difluoro-1-methyl propylene carbonate,
1,3,3-trifluoro-1-methyl propylene carbonate, 2-fluoro-1-methyl
propylene carbonate, 2,3-difluoro-1-methyl propylene carbonate,
2,3,3-trifluoro-1-methyl propylene carbonate,
3-fluoro-1-(fluoromethyl) propylene carbonate,
3,3-difluoro-1-(fluoromethyl) propylene carbonate,
1-fluoro-1-(fluoromethyl) propylene carbonate,
1,3-difluoro-1-(fluoromethyl) propylene carbonate,
2-fluoro-1-(fluoromethyl) propylene carbonate,
2,3-difluoro-1-(fluoromethyl) propylene carbonate,
3-fluoro-1-(difluoromethyl) propylene carbonate,
3,3-difluoro-1-(difluoromethyl) propylene carbonate,
1-fluoro-1-(difluoromethyl) propylene carbonate,
1,3-difluoro-1-(difluoromethyl) propylene carbonate,
2-fluoro-1-(difluoromethyl) propylene carbonate,
2,3-difluoro-1-(difluoromethyl) propylene carbonate and the
like.
[0029] Other examples of the additive A include instances where
R.sup.1 and R.sup.2 are a methyl group, a fluoromethyl group or a
difluoromethyl group, and all of R.sup.3, X.sup.1 and X.sup.2 are a
hydrogen atom or a fluorine atom. Examples of such additive A
include for instance 3-fluoro-1,1-dimethyl propylene carbonate,
3,3-difluoro-1,1-dimethyl propylene carbonate,
2-fluoro-1,1-dimethyl propylene carbonate,
2,3-difluoro-1,1-dimethyl propylene carbonate,
2,3,3-trifluoro-1,1-dimethyl propylene carbonate,
3-fluoro-1,1-bis(fluoromethyl) propylene carbonate,
3,3-difluoro-1,1-bis(fluoromethyl) propylene carbonate,
2-fluoro-1,1-bis(fluoromethyl) propylene carbonate,
2,3-difluoro-1,1-bis(fluoromethyl) propylene carbonate,
3-fluoro-1,1-bis(difluoromethyl) propylene carbonate,
3,3-difluoro-1,1-bis(difluoromethyl) propylene carbonate,
2-fluoro-1,1-bis(difluoromethyl) propylene carbonate,
2,3-difluoro-1,1-bis(difluoromethyl) propylene carbonate,
3-fluoro-1-fluoromethyl-1-methyl propylene carbonate,
3-fluoro-1-difluoromethyl-1-methyl propylene carbonate and the
like.
[0030] Other examples of the additive A include instances where all
of R.sup.1, R.sup.2 and R.sup.3 are a methyl group, a fluoromethyl
group or a difluoromethyl group, and both X.sup.1 and X.sup.2 are a
hydrogen atom or a fluorine atom. Examples of such additive A
include for instance 3-fluoro-1,1,2-trimethyl propylene carbonate,
3,3-difluoro-1,1,2-trimethyl propylene carbonate,
3-fluoro-1,1,2-tris(fluoromethyl) propylene carbonate,
3,3-difluoro-1,1,2-tris(fluoromethyl) propylene carbonate,
3-fluoro-1,1,2-tris(difluoromethyl) propylene carbonate,
3,3-difluoro-1,1,2-tris(difluoromethyl) propylene carbonate,
3-fluoro-1-fluoromethyl-1,2-dimethyl propylene carbonate,
3-fluoro-1-difluoromethyl-1,2-dimethyl propylene carbonate,
3-fluoro-1,2-bis(fluoromethyl)-1-methyl propylene carbonate and the
like.
[0031] Although not particularly limited thereto, the concentration
(content) C.sub.A of the additive A can be set to be 0.0001 mol/L
or higher. From the viewpoint of enhancing cycle durability, the
concentration C.sub.A of the additive A can be set to 0.0005 mol/L
or higher, 0.001 mol/L or higher, or even 0.002 mol/L or higher.
The above concentration C.sub.A may be for instance 0.1 mol/L or
higher, or 1 mol/L or higher. The upper limit of the concentration
C.sub.A of the additive A per 1 L of the nonaqueous electrolyte
solution is not particularly restricted, but the upper limit of the
concentration C.sub.A may be set to 15 mol/L or less, 10 mol/L or
less, or even 5 mol/L or less, for instance in terms of suppressing
rises in resistance. In some embodiments, the concentration C.sub.A
of the additive A per 1 L of the nonaqueous electrolyte solution
can be set to 0.001 mol/L to 3 mol/L, for instance to 0.025 mol/L
to 2.5 mol/L, or even from 0.025 mol/L to 1 mol/L.
[0032] Additive B
[0033] The nonaqueous electrolyte solution disclosed herein further
contains additive B represented by General Formula (II).
##STR00004##
Wherein R.sup.4 and R.sup.5 in General Formula (II) are selected,
each independently, from among halogen atoms and perfluoroalkyl
groups, and A.sup.+ is a cation.
[0034] The type of cation (A.sup.+) in the additive B is not
particularly limited and may be an organic cation or an inorganic
cation. Specific examples of inorganic cations include for instance
cations of alkali metals such as Li, Na and K; cations of alkaline
earth metals such as Be, Mg and Ca; cations of metals such as Ag,
Zn, Cu, Co, Fe, Ni, Mn, Ti, Pb, Cr, V, Ru and Y, lanthanoids and
actinoids; and protons. Examples of organic cations may include for
instance tetraalkylammonium ions such as tetrabutylammonium ions,
tetraethylammonium ions and tetramethylammonium ions;
trialkylammonium ions such as triethylmethylammonium ions and
triethylammonium ions; as well as pyridinium ions, imidazolium
ions, tetraethylphosphonium ions, tetramethylphosphonium ions,
tetraphenylphosphonium ions, triphenylsulfonium ions,
triethylsulfonium ions and the like. In some embodiments, the
cation in Additive B may include one or more than one of Li ions,
tetraalkylammonium ions, and protons.
[0035] In the additive B, substituents R.sup.4 and R.sup.5 on the
boron atom can be a halogen atom or a perfluoroalkyl group having 1
to 10, from 1 to 6, or even from 1 to 3 carbon atoms. Herein
R.sup.4 and R.sup.5 may be linear or branched. Further, R.sup.4 and
R.sup.5 may be identical or different. Examples of halogen atoms
include for instance fluorine atoms (F), chlorine atoms (Cl) and
bromine atoms (Br). In some embodiments, the halogen atoms may be
fluorine atoms. Examples of perfluoroalkyl groups having 1 to 10
carbon atoms include for instance perfluoromethyl groups,
perfluoroethyl groups, n-perfluoropropyl groups, perfluoroisopropyl
groups, n-perfluorobutyl groups, perfluoroisobutyl groups,
sec-perfluorobutyl groups, t-perfluorobutyl groups,
n-perfluoropentyl groups, perfluoropentyl groups, perfluorohexyl
groups, perfluoroheptyl groups, perfluorooctyl groups and the
like.
[0036] In one example of the additive B, both R.sup.4 and R.sup.5
are halogen atoms and the cation (A.sup.+) is a Li ion. Examples of
such additive B include for instance lithium difluorooxalatoborate
(LiDFOB), lithium dichlorooxalatoborate, lithium
dibromooxalatoborate and the like. In some embodiments, the
additive B may be lithium difluorooxalatoborate, which may improve
performance in terms of enhancing cycle durability.
[0037] Other examples of the additive B include instances where
both R.sup.4 and R.sup.5 are a perfluoroalkyl group having 1 to 10
carbon atoms, and the cation (A.sup.+) is a Li ion. Examples of
such additive B include for instance lithium
di(perfluoromethyl)oxalatoborate, lithium
di(perfluoroethyl)oxalatoborate, lithium
di(perfluoropropyl)oxalatoborate, lithium
di(perfluorobutyl)oxalatoborate and the like.
[0038] Other examples of the additive B include instances where one
from among R.sup.4 and R.sup.5 is a halogen atom and the other is a
perfluoroalkyl group having 1 to 10 carbon atoms, and the cation
(A.sup.+) is a Li ion. Examples of such additive B include for
instance lithium fluoro(perfluoromethyl)oxalatoborate, lithium
fluoro(perfluoroethyl)oxalatoborate, lithium
fluoro(perfluoropropyl)oxalatoborate, lithium
fluoro(perfluorobutyl)oxalatoborate and the like.
[0039] Although not particularly limited thereto, the concentration
(content) C.sub.B of the additive B can be set to be 0.0001 mol/L
or higher. From the viewpoint of enhancing cycle durability, the
concentration C.sub.B of the additive B may be set to 0.0005 mol/L
or higher, 0.001 mol/L or higher, or even 0.002 mol/L or higher.
The above concentration C.sub.B may be for instance 0.003 mol/L or
higher, or even 0.005 mol/L or higher. The upper limit of the
concentration C.sub.B of the additive B per 1 L of the nonaqueous
electrolyte solution is not particularly limited, but ordinarily
upper limit may be set to 3 mol/L or less, 1 mol/L or less, or even
0.5 mol/L or less, for instance in terms of suppressing rises in
resistance. In some embodiments, the concentration C.sub.B of the
additive B per 1 L of the nonaqueous electrolyte solution can be
set to 0.001 mol/L to 0.5 mol/L, for instance 0.025 mol/L to 0.1
mol/L.
[0040] The molar concentration C.sub.A of the additive A is higher
than the molar concentration C.sub.B of the additive B (i.e.
C.sub.A>C.sub.B) from the viewpoint of better bringing out the
effect derived from using concomitantly additive A and additive B.
A ratio (C.sub.A/C.sub.B) of the molar concentration C.sub.A of the
additive A with respect to the molar concentration C.sub.B of the
additive B may be about 1 or higher, for instance 2 or higher, or
even 5 or higher. For instance in terms of maintaining low
resistance, the above molar concentration ratio C.sub.A/C.sub.B may
be set to about 30 or less, 25 or less, 20 or less, 15 or less, or
even 10 or less. For instance, a nonaqueous electrolyte solution
having a molar concentration ratio C.sub.A/C.sub.B of 1 to 30 (in
particular 1 to 5) is appropriate from the viewpoint of combining
low resistance with enhanced cycle durability.
[0041] The nonaqueous electrolyte solution disclosed herein
contains the additive A and the additive B at a specific
concentration ratio. The cycle durability of a battery constructed
using a nonaqueous electrolyte solution containing a fluorinated
solvent can be effectively enhanced by using a combination of the
additive A and additive B, at a specific concentration ratio, in a
nonaqueous electrolyte solution. Conceivable underlying reasons for
such an effect include, although not meant to be limited thereto,
for instance the following. Specifically, fluorinated solvents have
low reduction resistance, and accordingly a dense coating film has
to be foiiiied that exhibits high electron insulating properties
and in which degradation can be suppressed, but the additive B does
not dissolve readily in fluorinated solvents, and a sufficient
coating film is not formed readily on a negative electrode surface.
Also, it is difficult to form a good coating film (coating film
having high electron insulating properties and in which degradation
can be suppressed) using the additive A singly. In a battery in
which the additive A and the additive B are used combined at a
specific concentration ratio, by contrast, the additive A and the
additive B decompose together with the electrolyte solution
components (fluorinated solvent, supporting salt and so forth) for
instance during initial charging, and a mixed coating film made up
of decomposition products of the foregoing, i.e. a mixed coating
film of the additive A and the additive B covers the surface of an
electrode (typically a negative electrode), so that as a result
further decomposition of electrolyte solution components is
suppressed, which can contribute to enhancing the performance of
the battery. Such a mixed coating film has higher electron
insulating properties and is denser than a coating film formed when
using the additive A or the additive B singly, and hence it becomes
possible to suppress effectively further decomposition of
electrolyte solution components. It is deemed that capacity
degradation and increases in resistance after endurance of the
battery can be effectively improved upon as a result.
[0042] Studies by the inventors on the basis of the test examples
described below have revealed that a higher performance improving
effect is achieved when the additive A and additive B are used in
combination at a specific concentration ratio, than when the
additive A and additive B are each used singly. In other words, a
nonaqueous electrolyte solution that allows enhancing cycle
durability can be provided by using additive A and additive B in
combination, at a specific concentration ratio, by virtue of the
synergy gained from such a combination.
[0043] The nonaqueous electrolyte solution disclosed herein may
contain a coating film forming agent (hereafter third additive)
other than the additive A and additive B. Examples of such a third
additive include for instance vinylene carbonate (VC),
monofluoroethylene carbonate (MFEC), 1,3-propane sultone (PS) and
the like. The amount of the third additive is for instance set to
50 mass % or less (for instance 0 mass % to 50 mass %), 35 mass %
or less, 20 mass % or less, or even 10 mass % or less, with respect
to the total mass of the coating film forming agent contained in
the nonaqueous electrolyte solution. The technology disclosed
herein can be implemented in a form where substantially no third
additive is present.
[0044] The nonaqueous electrolyte solution disclosed herein is thus
excellent in oxidation resistance and has good cycle durability,
and accordingly it can be used as a constituent element of lithium
ion secondary batteries of such a form. A lithium ion secondary
battery can be constructed in accordance with conventional
processes except that a fluorine-based solvent is used as a
nonaqueous solvent and that the additive A and additive B disclosed
herein are used. Although not meant to be particularly limited
thereto, an example of the lithium ion secondary battery
schematically illustrated in FIG. 1, will be explained as the
schematic configuration of a secondary battery provided with a
nonaqueous electrolyte solution according to the present
disclosure, but the scope of application of the present disclosure
is not meant to be limited to this example.
[0045] A lithium ion secondary battery 100 illustrated in FIG. 1
has a configuration in which a wound electrode body 80, of a fo in
resulting from winding flatly of a positive electrode sheet 10 and
a negative electrode sheet 20 across a separator sheet 40, is
accommodated in a box-shaped battery case 50 together with a
nonaqueous electrolyte solution not shown.
[0046] The battery case 50 is provided with a flat rectangular
parallelepiped shape (box type) battery case body 52 the top end of
which is open, and with a lid body 54 that plugs the opening of the
case body 52. A comparatively light metal (for instance, aluminum
or an aluminum alloy) can be used as the material of the battery
case 50. A positive electrode terminal 70 for external connection,
electrically connected to the positive electrode of the wound
electrode body 80, and a negative electrode terminal 72
electrically connected to the negative electrode of the wound
electrode body 80, are provided on the top face (i.e. lid body 54)
of the battery case 50. The lid body 54 is provided with a safety
valve 55 for discharging, out of the case 50, gas that is generated
inside the battery case 50, similarly to the battery cases of
conventional lithium ion secondary batteries.
[0047] The wound electrode body 80 of flat shape is accommodated,
together with the nonaqueous electrolyte solution (not shown)
described above, inside the battery case 50. The wound electrode
body 80 is provided with an elongated sheet-shaped positive
electrode (positive electrode sheet) 10 and an elongated
sheet-shaped negative electrode (negative electrode sheet) 20.
[0048] Positive Electrode
[0049] The positive electrode sheet 10 is provided with an
elongated positive electrode collector and a positive electrode
active material layer 14 formed along the longitudinal direction on
at least one surface or both surfaces of the positive electrode
collector. Such a positive electrode sheet 10 can be produced for
instance by applying a composition, resulting from dispersing a
forming component of a positive electrode active material layer in
an appropriate solvent (for instance N-methyl-2-pyrrolidone), onto
the surface of the positive electrode collector, and drying the
composition. The forming component of the above positive electrode
active material layer can contain a positive electrode active
material, and also for instance a conductive material and a binder
(binding agent) that are utilized as needed. A metal of good
conductivity (for instance, aluminum, nickel, titanium, stainless
steel or the like) can be used as the positive electrode
collector.
[0050] The operating upper limit potential of the positive
electrode of the lithium ion secondary battery disclosed herein is
4.3 V or higher, 4.35 V or higher, 4.6 V or higher, or even 4.7 V
or higher with reference to metallic lithium within a range of SOC
(State Of Charge) of 0% to 100%. The highest operating potential
between SOC 0% to 100% occurs generally at SOC 100%, and
accordingly the operating upper limit potential of the positive
electrode can be grasped ordinarily through the operating potential
of the positive electrode at SOC 100% (i.e. in a full charge
state). The technology disclosed herein can be used in a lithium
ion secondary battery in which the operating upper limit potential
of the positive electrode lies in the range of 4.3 V to 5.5 V (for
instance 4.7 V to 5.2 V) with reference to metallic lithium, within
a range of SOC 0% to 100%.
[0051] The positive electrode exhibiting such an operating upper
limit potential can be realized by using a positive electrode
active material having a highest value of operating potential of
4.3 V or higher (with respect to Li/Li.sup.+), for a SOC in the
range of 0% to 100%. Among the foregoing there is used a positive
electrode active material the operating potential of which, at SOC
100%, exceeds 4.3 V, 4.5 V or higher, 4.6 V or higher, or even 4.9
V or higher, with reference to metallic lithium. Yet higher energy
density can be realized by using a positive electrode active
material having the above operating potential. Side reactions with
a positive electrode can be suitably suppressed through the use, in
the nonaqueous electrolyte solution, of the fluorinated solvent,
also in a positive electrode at such high potential.
[0052] The operating potential of a positive electrode active
material can be measured for instance as follows. Specifically,
firstly there is constructed a tri-polar cell using a working
electrode (WE) in the form of a positive electrode containing a
positive electrode active material, as the measurement target, as
well as a counter electrode (CE) and metallic lithium as a
reference electrode (RE), and a nonaqueous electrolyte solution.
Next, the SOC of the cell is adjusted from 0% to 100% in increments
of 5%, on the basis of the theoretical capacity of the cell.
Adjustment of the SOC can be accomplished through constant-current
charging across the WE-CE, using for instance an ordinary charging
and discharging device or a potentiostat. The cell having been
adjusted to a given SOC state is allowed to stand for 1 hour, after
which potential across the WE-RE is measured, and the potential is
taken as the operating potential (with respect to Li/Li.sup.+) of
the positive electrode active material at the respective SOC
state.
[0053] Examples of positive electrode active materials that can
suitably realize such high potential include for instance
lithium-manganese complex oxides of spinel structure. Some
embodiments among the foregoing include lithium-nickel-manganese
complex oxides of spinel structure having Li, Ni and Mn as
constituent metal elements. A more specific example is a
lithium-nickel-manganese complex oxide having a spinel structure
and represented by General Formula (III).
Li.sub.x(Ni.sub.yMn.sub.2-y-zMe.sup.1.sub.z)O.sub.4+.alpha.
(III)
Wherein Me.sup.1 can be any transition metal element or typical
metal element (for instance one, two or more elements selected from
among Fe, Ti, Co, Cu, Cr, Zn and Al), other than Ni and Mn.
Alternatively, Me.sup.1 may be a metalloid element (for instance
one, two or more elements selected from among B, Si and Ge) or a
non-metal element. Further, x is 0.8.ltoreq.x.ltoreq.1.2; y is
0<y; z is 0.ltoreq.z; there holds y+z<2, or even
y+z.ltoreq.1; and .alpha. is a value such that a charge neutral
condition is satisfied with -0.2.ltoreq..alpha..ltoreq.0.2. In some
embodiments, y is 0.2.ltoreq.y.ltoreq.1.0, such as
0.4.ltoreq.y.ltoreq.0.6, for instance 0.45.ltoreq.y.ltoreq.0.55;
and z is 0.ltoreq.z<1.0 (for instance 0.ltoreq.z.ltoreq.0.3).
Examples of the lithium-nickel-manganese complex oxide represented
by the general formula above include for instance
LiNi.sub.0.5Mn.sub.1.5O.sub.4 and the like. Such
lithium-nickel-manganese complex oxides of spinel structure can
contribute to increasing energy density. Whether a compound (oxide)
has a spinel structure or not can be determined for instance by
X-ray structural analysis (preferably single-crystal X-ray
structural analysis). Specifically, the presence or absence of a
spinel structure can be determined by X-ray diffraction
measurements using CuK.alpha. rays.
[0054] Other examples of the positive electrode active material
disclosed herein include for instance lithium transition metal
complex oxides, typically of layered structure, represented by
general formula LiMe.sup.2O.sub.2. Herein Me.sup.2 includes at
least one transition metal element such as a Ni, Co, Mn and the
like, and can further include other metal elements or non-metal
elements. Such a layered-structure lithium transition metal complex
oxide can contribute to increasing the capacity of the battery.
[0055] Other examples of the positive electrode active material
disclosed herein include for instance lithium transition metal
compounds (phosphate salts) of olivine structure represented by
foiliiula LiMe.sup.3PO.sub.4. Herein Me.sup.3 includes at least one
transition metal element such as Mn, Fe, Co and the like, and can
further include other metal elements or non-metal elements.
Examples include for instance LiMnPO.sub.4, LiFePO.sub.4,
LiCoPO.sub.4 and the like.
[0056] Other examples of the positive electrode active material
disclosed herein include of solid solutions of LiMe.sup.2O.sub.2
and Li.sub.2Me.sup.4O.sub.3. Herein LiMe.sup.2O.sub.2 represents a
composition represented by the general formula above. Further,
Me.sup.4 in Li.sub.2Me.sup.4O.sub.3 includes at least one
transition metal element such as Mn, Fe, Co and the like, and can
further include other metal elements or non-metal elements.
Examples include for instance Li.sub.2MnO.sub.3 or the like.
Examples of the above solid solution include for instance a solid
solution represented by
0.5LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2-0.5Li.sub.2MnO.sub.3.
[0057] The positive electrode active material described above can
be used singly or as a combination of two or more types. Among the
foregoing, in some embodiments, the positive electrode active
material contains a lithium-nickel-manganese complex oxide of
spinel structure represented by General Formula (III) in a
proportion of 50 mass % or higher, such as 50 mass % to 100 mass %,
for instance 70 mass % to 100 mass %, or even 80 mass % to 100 mass
% with respect to the total positive electrode active material that
is used. In some embodiments, the positive electrode active
material is substantially made up of only a
lithium-nickel-manganese complex oxide of spinel structure.
[0058] In the technology disclosed herein the positive electrode
active material is of particulate form having an average particle
size of 1 .mu.m to 20 .mu.m, or from 2 .mu.m to 15 .mu.m. Unless
otherwise specified the term "average particle size" denotes a
particle size (D.sub.50, also referred to as median size)
corresponding to a cumulative frequency of 50 vol %, from the side
of microparticles of small particle size in a volume-based particle
size distribution based on a laser diffraction/light scattering
method.
[0059] Other Constituent Components of the Positive Electrode
Active Material Layer
[0060] Besides the positive electrode active material, the positive
electrode active material layer can contain additives such as a
conductive material and a binder (binding material). A conductive
powder material such as carbon powder or carbon fibers may be used
as the conductive material. Examples of carbon powder include for
instance various carbon blacks (CB) for instance acetylene black
(AB).
[0061] Examples of the binder include various polymer materials.
For instance a water-soluble or water-dispersible polymer material
can be used in a case where a positive electrode active material
layer is formed using an aqueous composition (composition in which
a dispersion medium is water or a mixed solvent having water as a
main component). Examples of water-soluble and water-dispersible
polymer materials include for instance cellulosic polymers such as
carboxymethyl cellulose (CMC); fluororesins such as
polytetrafluoroethylene (PTFE); and rubbers such as styrene
butadiene rubber (SBR). Alternatively, a polymer material, for
instance a halogenated vinyl resin such as polyvinylidene fluoride
(PVdF); or a polyalkylene oxide such as polyethylene oxide (PEO)
can be used in a case where the positive electrode active material
layer is formed using a solvent-based composition (composition in
which dispersion medium is mainly an organic solvent). Such a
binder can be used singly or as a combination of two or more types.
In addition to being used as a binder, the polymer materials
exemplified above may be used as a thickener, a dispersant or as
some other additive.
[0062] The proportion of the positive electrode active material in
the positive electrode active material layer as a whole is about 50
mass % or more, such as 50 mass % to 95 mass %. In some
embodiments, the proportion is set to 70 mass % to 97 mass %, such
as from 75 mass % to 95 mass %. If a conductive material is used,
the proportion of the conductive material in the positive electrode
active material layer as a whole can be set to about 2 mass % to 20
mass %. In some embodiments, the proportion is set to about 2 mass
% to 15 mass %. If a binder is used, the proportion of the binder
in the positive electrode active material layer as a whole can be
set to about 0.5 mass % to 10 mass, such as from about 1 mass % to
5 mass %.
[0063] Negative Electrode
[0064] The negative electrode sheet 20 is provided with an
elongated negative electrode collector and with a negative
electrode active material layer 24 formed along the longitudinal
direction on at least one surface or both surfaces of the negative
electrode collector. Such a negative electrode sheet 20 can be
produced for instance by applying, onto the surface of the negative
electrode collector, a composition resulting from dispersing a
forming component of a negative electrode active material layer in
an appropriate solvent (for instance water), and by drying the
composition. The forming component of the negative electrode active
material layer can contain a negative electrode active material,
and a binder and so forth that are used as needed. A conductive
material made up of a metal of good conductivity (for instance,
copper, nickel, titanium, stainless steel or the like) can be used
as the negative electrode collector.
[0065] Herein, one, two, or more substances that are utilized in
lithium ion secondary batteries can be used, without particular
limitations, as the negative electrode active material. Examples of
the negative electrode active material include for instance carbon
materials. Examples of carbon materials include for instance
graphite carbon (graphite), amorphous carbon, and the like. A
particulate carbon material (carbon particles) containing a
graphite structure (layered structure) at least partially may be
used in some embodiments. In other embodiments, a carbon material
having natural graphite as a main component may be used among the
foregoing. The above natural graphite can be obtained through
spheroidization of scaly graphite. A carbonaceous powder resulting
from coating the surface of graphite with amorphous carbon may also
be used herein. As the negative electrode active material there can
be alternatively used single species, alloys and compounds of metal
oxide materials such as silicon oxide, titanium oxide, vanadium
oxide and lithium-titanium composite oxides (LTO); metal nitride
materials such as lithium nitride, lithium-cobalt complex nitrides,
lithium-nickel complex nitrides; and also, silicon materials, tin
materials and the like, as well as composite materials in which the
foregoing materials are used concomitantly. A negative electrode
active material having a reduction potential (with respect to
Li/Li.sup.+) of about 0.5 V or less, such as 0.2 V or less, or even
0.1 V or less, may be used among the foregoing. Higher energy
density can be realized by using a negative electrode active
material having the above reduction potential. Examples of
materials that can yield such low potential include for instance
natural graphite-based carbon materials. In the technology
disclosed herein the negative electrode active material may be of
particulate form having an average particle size of 10 .mu.m to 30
.mu.m, such as from 15 .mu.m to 25 .mu.m.
[0066] Other Constituent Components of the Negative Electrode
Active Material Layer
[0067] Besides the negative electrode active material, the negative
electrode active material layer can contain additives such as a
binder (binding agent), a thickener and the like. The same
materials explained regarding the positive electrode active
material layer can be used as the binder and the thickener that are
used in the negative electrode active material layer.
[0068] The proportion of the negative electrode active material in
the negative electrode active material layer as a whole exceeds
about 50 mass %, such as from about 80 mass % to 99.5 mass %, or
even 90 mass % to 99 mass %. The proportion of binder in the
negative electrode active material layer as a whole may be about
0.5 mass % to 5 mass %, such as from mass % to 2 mass %. The
proportion of thickener in the negative electrode active material
layer as a whole may be about 0.5 mass % to 5 mass %, such as from
1 mass % to 2 mass %.
[0069] Two separators (separator sheets) 40 having an elongated
sheet shape are disposed, between the positive electrode active
material layer 14 and the negative electrode active material layer
24, as an insulating layer for preventing direct contact between
the foregoing. A porous sheet made of a resin such as polyethylene
(PE), polypropylene (PP), polyester, cellulose, polyamide or the
like, or a nonwoven fabric or the like, can be used as the
separator sheets 40.
[0070] The wound electrode body 80 can be produced for instance by
winding, in the longitudinal direction, a stack resulting from
sequentially superimposing the positive electrode sheet 10, a
separator sheet 40, the negative electrode sheet 20 and a separator
sheet 40, and by pressing and squashing the resulting wound body
from the sides, to form the stack into a flat shape.
[0071] A wound core portion in which the positive electrode active
material layer 14 formed on the surface of the positive electrode
collector and the negative electrode active material layer 24
formed on the surface of the negative electrode collector are
superimposed and densely overlaid on each other is formed at a
central portion of the wound electrode body 80 in the width
direction, defined as the direction from one end of the wound
electrode body 80 towards the other end in the winding axis
direction. A positive electrode active material layer non-forming
portion of the positive electrode sheet 10 and a negative electrode
active material layer non-forming portion of the negative electrode
sheet 20 jut outward of the wound core portion, on both ends of the
wound electrode body 80 in the winding axis direction. A positive
electrode power collector plate is attached to the jutting portion
on the positive electrode side and a negative electrode power
collector plate is attached to the jutting portion on the negative
electrode, and the collector plates are electrically connected to
the positive electrode terminal 70 and the negative electrode
terminal 72, respectively.
[0072] Method for Producing a Lithium Ion Secondary Battery
[0073] The lithium ion secondary battery 100 having such a
configuration can be produced as a result of a battery assembly
construction step and an initial charging step.
[0074] Battery Assembly Construction Step
[0075] In the battery assembly construction step the wound
electrode body 80 provided with the positive electrode sheet 10 and
the negative electrode sheet 20 is accommodated inside a battery
case together with the nonaqueous electrolyte solution, to
construct thereby a battery assembly. The term battery assembly
denotes a battery having been assembled to the form that precedes
the initial charging step in the production processes of the
battery. The battery assembly can be constructed for instance by
accommodating the wound electrode body 80 inside the battery case
50, through the opening of the battery case 50, attaching the lid
body 54 to the opening of the case 50, injecting thereafter a
nonaqueous electrolyte through an injection hole, not shown,
provided in the lid body 54, and sealing next the injection hole by
welding or the like. In this implementation the nonaqueous
electrolyte solution accommodated in the battery assembly contains
the above fluorinated solvent as the nonaqueous solvent. The
nonaqueous electrolyte solution contains the additive A represented
by General Formula (I) and the additive B represented by General
Formula (II).
[0076] Initial Charging Step
[0077] In the initial charging step the battery assembly is
subjected to initial charging. Typically, an external power source
is connected between the positive electrode (positive electrode
terminal) and the negative electrode (negative electrode terminal)
of the battery assembly, to charge the battery assembly (e.g., by
constant-current charging) up to a predetermined voltage range. A
good-quality coating film containing components derived from the
additive A and the additive B becomes formed as a result on the
negative electrode surface.
[0078] The voltage during initial charging may be for instance set
such that the additive A and the additive B are electrically
decomposed. As an example, the negative electrode active material
is a carbon material, charging is performed up to a voltage of
about 3 V or higher (3.5 V or higher, or even 4.7 V or higher),
such as from 4 V to 5 V, across the positive and negative electrode
terminals. Such charging may be performed according to a scheme (CC
charging) that involves constant-current charging from the start of
charging until the battery voltage reaches a predetermined value,
or according to a scheme (CC-CV charging) in which constant-voltage
charging is performed after the above predetermined voltage has
been reached. The charging rate at the time of constant-current
charging is ordinarily 1 C or lower, for example from 0.1 C to 0.2
C. Findings by the inventors have revealed that the additive A and
the additive B decompose relatively gently when charging is
performed at a low rate of 1 C or less. A coating film containing
components of the additive A and the additive B is formed, with
suitable denseness (for instance, a coating film of low resistance
and capable of sufficiently suppressing reactivity with a
nonaqueous electrolyte solution), on the surface of the negative
electrode. The effect of the present configuration can therefore be
brought out to a yet higher level. Charging may be perfoinied once,
or twice or more, for instance with intervening discharges.
[0079] The lithium ion secondary battery 100 according to the
present embodiment can be thus produced in the above manner.
[0080] Although the lithium ion secondary battery disclosed herein
can be used in various applications, the characterizing feature of
the battery is its excellent cycle durability, and hence the
battery can be used, by exploiting that characterizing feature, in
applications where high performance (for instance long lifespan) is
required. Relevant applications include for instance driving power
sources that are installed in vehicles such as plug-in hybrid
automobiles, hybrid automobiles, electric automobiles and the like.
Such secondary batteries can be used in the form of assembled
batteries resulting from series and/or parallel connection of a
plurality of secondary batteries.
[0081] Several examples relating to the present disclosure will be
described below, but the disclosure is not intended to be limited
to such examples.
[0082] Nonaqueous Electrolyte Solution
Example 1
[0083] A reference electrolyte solution NA, being a solution
containing LiPF.sub.6 at a concentration of 1 mol/L in a mixed
solvent of trifluoropropylene carbonate (TFPC) and methyl
trifluoroethyl carbonate (MTFEC) (volume ratio 30:70), as a
fluorinated solvent, was used as an electrolyte solution sample of
Example 1.
Example 2
[0084] An electrolyte solution sample of Example 2 was obtained by
adding 3-fluoropropylene carbonate (FPC: compound where R.sup.1,
R.sup.2 and R.sup.3=H, and R.sup.4=CFH.sub.2 in General Formula
(I)), as the additive A, to the reference electrolyte solution NA,
to a concentration C.sub.A of 0.5 mol/L.
Example 3
[0085] An electrolyte solution sample of Example 3 was obtained by
adding lithium difluorooxalatoborate (LiDFOB: compound
corresponding to R.sup.4 and R.sup.5=F, and A=Li in General Formula
(II)), as the additive B, to the reference electrolyte solution NA
to a concentration C.sub.B of 0.05 mol/L.
Examples 4 to 14
[0086] Electrolyte solution samples of Examples 4 to 14 were
obtained by adding FPC as the additive A and LiDFOB as the additive
B to the reference electrolyte solution NA, to respective
predetermined concentrations C.sub.A, C.sub.B.
[0087] Table 1 summarizes the types and concentration C.sub.A of
the additive A, the types and concentration C.sub.B, as well as the
concentration ratio C.sub.A/C.sub.B, for the electrolyte solution
samples in the various examples.
TABLE-US-00001 TABLE 1 Additive A Additive B Concentration Capacity
Concentration Concentration ratio retention Resistance Example Type
C.sub.A (mol/L) Type C.sub.B (mol/L) CA/CB rate (%) (.OMEGA.) 1 --
-- -- -- -- 68 1.4 2 FPC 0.5 -- -- -- 63 1.6 3 -- -- LiDFOB 0.05 --
77 1.3 4 FPC 0.025 LiDFOB 0.05 0.5 76 1.3 5 FPC 0.5 LiDFOB 0.01 50
70 1.6 6 FPC 3 LiDFOB 0.1 30 88 2 7 FPC 0.05 LiDFOB 0.05 1 91 1.3 8
FPC 0.1 LiDFOB 0.05 2 91 1.3 9 FPC 0.025 LiDFOB 0.025 1 87 1.4 10
FPC 0.5 LiDFOB 0.025 20 88 1.5 11 FPC 0.5 LiDFOB 0.05 10 90 1.4 12
FPC 0.5 LiDFOB 0.1 5 89 1.4 13 FPC 1 LiDFOB 0.1 10 89 1.5 14 FPC
2.5 LiDFOB 0.1 25 90 1.7
[0088] Production of Lithium Ion Secondary Batteries
[0089] Lithium ion secondary batteries for evaluation were produced
in the manner described below, using the respective electrolyte
solutions produced in Examples 1 to 14.
[0090] The positive electrode of the lithium ion secondary
batteries was produced as follows. Specifically, a composition for
foiiiiing a positive electrode active material layer was prepared
by mixing a spinel structure lithium-nickel-manganese complex oxide
(LiNi.sub.0.5Mn.sub.1.5O.sub.4), as a positive electrode active
material, acetylene black (AB) as a conductive material, and PVdF
as a binder, to a mass ratio of the foregoing materials of 87:10:3.
This composition was applied onto one side of an aluminum foil
(positive electrode collector), and was dried, followed by
roll-pressing to a density of the positive electrode active
material layer of 2.3 g/cm.sup.3, to thereby obtain a positive
electrode sheet having a positive electrode active material layer
formed on the positive electrode collector.
[0091] The negative electrode of the lithium ion secondary
batteries was produced as follows. Specifically, a composition for
forming a negative electrode active material layer was prepared by
mixing, in water, natural graphite (average particle size 10 .mu.m,
surface area D.sub.50=4.8 m.sup.2/g) as a negative electrode active
material, SBR as a binder and CMC as a thickener to a mass ratio of
the foregoing materials of 98:1:1. This composition was applied
onto one side of a copper foil (negative electrode collector), and
was dried, followed by roll-pressing to thereby obtain a negative
electrode sheet having a negative electrode active material layer
formed on a negative electrode collector. The coating amounts of
the composition for forming a positive electrode active material
layer and of the composition for forming a negative electrode
active material layer were adjusted to yield a ratio by weight of
2:1 of the positive electrode active material and the negative
electrode active material.
[0092] As a separator there was prepared a separator made up of a
base material of a micro-porous membrane (PP/PE/PP membrane) having
a three-layer structure of
polypropylene/polyethylene/polypropylene.
[0093] A laminate cell was constructed using the positive
electrode, negative electrode and separator having been prepared
above. Specifically, electrode bodies were produced by
superimposing the produced positive and electrode negative
electrode, with the separator interposed in between, in such a
manner that the active material layers of the electrodes opposed
each other. Next, the electrode bodies were accommodated in
laminate-made bag-like battery containers, together with the
electrolyte solution samples of the examples, to construct
respective battery assemblies.
[0094] Each battery assembly was conditioned by repeating thrice an
operation that involved charging at C/3 constant current, at a
temperature of 25.degree. C., until 4.9 V was reached, followed by
a 10-minute pause, with subsequent discharge at C/3 constant
current down to 3.5 V, followed in turn by a 10-minute pause.
Respective evaluation cells (laminate cells) of Examples 1 to 14
were constructed in this manner.
[0095] Measurement of Rated Capacity
[0096] Each evaluation cell was charged at C/3 constant current, at
a temperature of 25.degree. C., until 4.9 V was reached, with
subsequent charging for 2.5 hours at the same voltage. After a
10-minute pause, the cell was discharged at a C/3 rate down to 3 V,
with subsequent discharge for 2 hours at the same voltage; the
discharge capacity measured at the time of this discharge was taken
herein as the initial capacity (rated capacity).
[0097] Measurement of Initial Resistance
[0098] The evaluation cells of the examples, having been adjusted
to SOC 60% (charge state of about 60% of the rated capacity) were
discharged at rates of 1 C, 3 C, 5 C and 10 C, for 10 seconds, and
the voltage drops during that lapse of time were measured. Internal
resistance was calculated by dividing the measured voltage drop
amount by the current value during discharge, and the calculated
internal resistance was taken as the initial resistance. The
results are given in Table 1 in the column "Resistance".
[0099] Cycle Endurance Test
[0100] The evaluation cells of the examples were placed in a
thermostatic bath at about 60.degree. C., and were subjected to a
cycle endurance test that involved repeating continuously 200
charge and discharge cycles, each including charging at 2 C of
constant current up to 4.9 V, followed by discharge at 2 C of
constant current down to 3.5 V.
[0101] Capacity retention rates were calculated on the basis of the
initial capacity before the cycle endurance test and the battery
capacity after the cycle endurance test. The battery capacity after
the cycle endurance test was measured in accordance with the same
procedure as that of the initial capacity described above. The
capacity retention rate was worked out as "battery capacity after
cycle endurance test/initial capacity before cycle endurance
test".times.100. The results are given in Table 1 in the column
"Capacity retention rate".
[0102] As Table 1 reveals, the evaluation cell of Example 2, which
utilized a nonaqueous electrolyte solution having only FPC added
thereto, exhibited a drop in capacity retention rate after the
endurance test as compared with Example 1, in which a nonaqueous
electrolyte solution without additive was used. The evaluation cell
of Example 3, in which there was used a nonaqueous electrolyte
solution having only LiDFOB added thereto, exhibited a slight
improvement in capacity retention rate as compared with Example 1,
but the value of capacity retention rate was lower than 80%. The
capacity retention rates of the evaluation cells of Examples 4 and
5, in which FPC and LiDFOB were used concomitantly and the
concentration ratio C.sub.A/C.sub.B was set to 0.05 and 50,
exhibited a slight improvement as compared with Example 1, but the
values of capacity retention rate were lower than 80%. The
evaluation cells of Examples 6 to 14, in which FPC and LiDFOB were
used concomitantly and the concentration ratio C.sub.A/C.sub.B was
set to 1 to 30, exhibited by contrast good durability in that it
was possible to achieve a capacity retention rate of 88% or higher
after the endurance test. The above indicated that a high
durability enhancing effect can be achieved by using concomitantly
the additive A and the additive B, and by setting the concentration
ratio C.sub.A/C.sub.B to 1 to 30. The evaluation cells of Examples
5 to 14, in which FPC and LiDFOB were used concomitantly and the
concentration ratio C.sub.A/C.sub.B was set to 1 to 25, afforded a
lower resistance than that of Example 6, in which FPC and LiDFOB
were used concomitantly and the concentration ratio C.sub.A/C.sub.B
was set to 30. The concentration ratio C.sub.A/C.sub.B is thus set
to 25 or less from the viewpoint of maintaining low resistance.
[0103] Examples of the present disclosure have been explained in
detail above, but these examples are merely illustrative in nature,
and are not meant to limit the scope of the claims. The present
disclosure may encompass various modifications and alterations of
the above-described examples.
* * * * *